Chemical imbalances in the human body are responsible for changes in mood and behaviour of a person. Antidepressants are a class of drugs that reduce symptoms of depressive disorders by correcting chemical imbalances of neurotransmitters in the brain. They are usually used to treat several conditions for example depression, generalized anxiety disorder, agitation, obsessive compulsive disorders and the like. Most often prescribed antidepressant drugs include Venlafaxine which are commercially available under the tradename Effexor, Lanvexin, Viepax and Trevilor. 1-[2-(Dimethylamino)-1-(4-methyloxyphenyl) ethyl] cyclohexanol, commonly known as Venlafaxine is an antidepressant of the serotonin-norepinephrine reuptake inhibitor (SNRI) class. Venlafaxine acts by inhibiting re-uptake of norepinephrine and serotonin, and is an alternative to the tricyclic antidepressants and selective re-uptake inhibitors. Venlafaxine is of the structural formula (I)

However due to profound side-effects observed, in the recent past Venlafaxine is not recommended as a first line of treatment. Also, Venlafaxine is extensively metabolized producing several metabolites.
However it is found that, [1-[2-(Dimethylamino)-1-(4-hydroxphenyl)ethyl]-cyclohexanol], commonly known as Desvenlafaxine is also an effective serotonin-norepinephrine reuptake inhibitor (SNRI) antidepressant and a safe treatment for major depressive disorders (TDD). Desvenlafaxine differs from Venlafaxine in that it is deficient of a methyl group attached to the phenolic ring. The structural formula of Desvenlafaxine (formula II) is as follows:

Also, Desvenlafaxine is less extensively metabolized, when compared to Venlafaxine, with absolute bioavailability. Desvenlafaxine is administered as its salt Desvenlafaxinesuccinate and also an active metabolite of Venlafaxine.
Patent literature discloses the synthesis of Desvenlafaxine by the process of demethylation, of Venlafaxine using thiols, metal sulphides, lithium diphenylphosphide, alkyl aluminium hydride and the like. However these methods involve heavy metal catalyst, large volumes of solvents, and expensive reagents and very smelling sulphur reagents. Because of the odor during the manufacturing, special precautions and scrubbing is required for large scale manufacturing.
U.S. Pat. No. 4,535,186 discloses synthesis of Desvenlafaxine and its pharmaceutically acceptable salts specifically fumarate salt. The invention discloses a process for the preparation of desmethylvenlafaxine that involves use of a benzyl blocking group on the 4-hydroxy group of the phenyl ring.
U.S. Pat. No. 7,491,848 discloses synthesis of Desvenlafaxine, formed from Venlafaxine by the use of a demethylating agent comprising a metal sulfide, such as sodium sulfide,
U.S. Pat. No. 6,689,912 discloses a method of preparing Desmethylvenlafaxine which comprises reacting Venlafaxine with a high molecular weight alkane or arene thiolate anion in an alcohol, ethylene glycol, ether of ethylene glycol, or mixture thereof, to provide desmethylvenlafaxine.
US20050197392 discloses a method for the preparation of desmethylvenlafaxine which involves demethylation of Venlafaxine using Lithium diphenylphosphide. The method involves extraction steps involving large volumes of solvent.
WO2010079046 discloses a process for the preparation of Desvenlafaxine and its salts, based on the demethylation of Venlafaxine by the use of alkyl aluminium or alkyl aluminium hydride compounds, particularly Di-isobutylaluminium hydride or tri-isobutylaluminium.
WO2007120923 provides a process for preparing Desvenlafaxine comprising-combining under reduced pressure venlafaxine, an organic solvent and a reagent consisting of: thiophenol, or sodium sulfide or Ci-Cs alkyl thiolate.
The aforementioned processes generally proceed with many side reactions thereby affecting the yield of Desvenlafaxine. They are complex and environmentally hazardous process. The reactions also emanate foul odour polluting the environment. The reagents used for the preparation are expensive and also the production of Desvenlafaxine on large scale is difficult and tedious.
Hence there is a need for an alternative economical, eco-friendly method of preparation of Desvenlafaxine. The process described in the present invention implements a simple, cost effective method of synthesis of Desvenlafaxine and also the salt of Desvenlafaxine by eliminating the use of expensive reagents.